1. Field of the Invention
The present invention relates to an exposure apparatus for exposing and transferring, e.g., a reticle pattern onto a photosensitive substrate in the photolithography process for manufacturing a device such as a semiconductor device, an image sensing device such as a CCD (Charge-Coupled Device), a liquid crystal display device, or a thin-film magnetic head, and a positioning apparatus and, more particularly, to an exposure apparatus having a parallel link mechanism capable of supporting a semiconductor wafer, accurately positioning it, and conveying it at high speed.
2. Description of the Related Art
The structure of the positioning mechanism of a conventional semiconductor exposure apparatus will be described with reference to FIG. 7. Referring to FIG. 7, reference numeral 102 denotes a fine moving stage mounted on an X stage 110 to finely position a semiconductor wafer 103; 104, a Y stage; 105R and 105L, moving magnet type movable elements of a linear motor for position-controlling the Y stage 104; 106R and 106L, coils as the stators of the linear motor; 107, a stage base plate; and 108, a main body base plate supported by active support legs 109a to 109d constituting an active anti-vibration device. The support legs 109b, 109c, and 109d are arranged at three corners of the main body base plate 108, other than the corner with the support leg 109a, although they are not illustrated in FIG. 7.
An X-Y stage 101 will be described first. The X stage 110 receives a thrust from a linear motor (not shown) and is position-controlled in the X direction of a coordinate system shown in FIG. 7 on the basis of a position signal measured by irradiating a moving mirror 111X with a laser beam from a laser interferometer. The Y stage 104 is applied with a thrust by a current supplied from the movable elements 105R and 105L of linear motors disposed on the left and right sides to the coils 106R and 106L, and is position-controlled in the Y direction of the coordinate system on the basis of a position signal measured by irradiating a moving mirror 111Y with a laser beam from a laser interferometer.
The fine moving stage 102 will be described next. Three actuators (not shown) for generating thrusts vertically in the Z direction and an actuator for rotating the fine moving stage 102 about the Z-axis are arranged to construct a mechanism for controlling translation in the direction of the optical axis, i.e., Z-axis of the projection optical system, rotation about the X- and Y-axes as the coordinate axes which are located in a horizontal plane perpendicular to the optical axis, and rotation about the Z-axis. As the actuator, a piezoelectric element or a linear motor is preferably used. A position sensor for positioning the fine moving stage 102 is placed at the periphery under the fine moving stage 102, though it is not illustrated.
As described above, with the X-Y stage 101 and fine moving stage 102, the semiconductor wafer 103 can be moved in the two-dimensional plane at a high speed and set at a desired position, and then, focusing along the optical axis of a projecting lens (not shown) and leveling drive at a tilt with respect to the optical axis can be performed. More specifically, the X-Y stage 101 performs positioning with a total of two-degrees-of-freedom, i.e., X and Y translation operations, and the fine moving stage 102 performs positioning with a total of four-degrees-of-freedom, i.e., Z translation along the optical axis, rotation about the X-axis, rotation about the Y-axis, and rotation about the Z-axis. That is, motion with a total of six-degrees-of-freedom is controlled.
Recently, a mechanism called a parallel link mechanism has been introduced. It has received a great deal of attention as a mechanism having high rigidity and capable of high-speed drive, as compared to a serial link scheme used for a conventional robot arm. For example, a machine tool requires a drive mechanism having high rigidity to withstand working disturbance. Employment of parallel link mechanisms to such tools starts aiming at realization of high-speed working. Practical industrial robots which can be used for welding, assembly, handling, or the like have been produced recently. These are known industrial robots described in Uchiyama, Sadoyu, and Masukawa, "Dynamic Control Experiments of Parallel Robot HEXA", Transactions of Robotics Society of Japan, Vol. 14, No. 2, pp. 143-150 (1996).
FIG. 8 shows an example of the parallel link mechanism. The mechanical structure and operation will be described.
Referring to FIG. 8, reference numeral 119 denotes a movable plate; 120, a fixed plate; 121, spherical bearings attached to the movable and fixed plates, respectively; and 122, a extendible actuator unit. As the extendible actuator 122, a hydraulic actuator, an air cylinder, a combination of a motor and a ball screw, or a motor, reduction gears, and a ball screw, or a linear motor is used. The spherical bearings 121 and actuator 122 unit construct a link 123. In the parallel link mechanism shown in FIG. 8, the movable plate 119 and fixed plate 120 are connected with a total of six links. By controlling the extension/retraction amount of each link 123, the posture of the coordinate system (x.sub.1,y.sub.1,z.sub.1) of the movable plate 119, which has six-degrees-of-freedom, with respect to the coordinate system (x.sub.2,y.sub.2,z.sub.2) defined for the fixed plate 120 can be arbitrarily set.
The actuator unit 122 of each link 123 incorporates a standard position sensor (not shown). In addition, each link may have at least one of a speed sensor and an acceleration sensor as needed.
A conventional exposure apparatus has an X-Y stage for moving a semiconductor wafer in a horizontal two-dimensional plane at a high speed to set the wafer at a predetermined position and a focus/leveling stage, i.e., a fine moving stage for moving the semiconductor wafer in the direction of the optical axis of a projecting lens and adjusting the tilt. That is, in the conventional exposure apparatus, the heavy X-Y and fine moving stages are driven to convey a semiconductor wafer with a small weight of several hundred grams. Since the heavy and complex mechanism is driven as a whole, it can be understood that a very inefficient motion mechanism is used from the viewpoint of energy consumption. To achieve a higher-speed operation in the conventional exposure apparatus, the thrust of the linear motor (FIG. 7) needs to be increased. However, an increase in thrust results in a bulky drive mechanism and inevitably poses a serious problem of heat generation.
In a conventional parallel link mechanism, a position sensor or the like is arranged in each actuator unit. The movable plate to be positioned by the parallel link mechanism is feedback-controlled on the basis of the sensor output. However, when an object mounted on the movable plate is to be accurately positioned, e.g., when positioning is to be performed on the submicrometer order, it is difficult to accurately position the object on the movable plate by only positioning using the sensors of the actuators.